Halogen-containing gases are environmental hazards and must be removed or reduced from emission sources. Treatment of fluorine gas (F2) is especially problematic since it is only marginally soluble in water and, thus, cannot be efficiently removed from an effluent stream via water scrubbing. The solubility in water is also poor for other halogen-containing gases such as trichloroethylene, chloroform, perchloroethylene, various chlorofluorocarbons (“CFCs”), and various perfluorinated carbons (“PFCs”). Effluent streams from semiconductor manufacturing often contain such halogen-containing gases. F2 is of particular interest since it is becoming more common as an emission product from NF3-based dielectric chamber cleaning processes.
Conventional treatment of F2 gas involves combustion with a fuel gas (e.g., natural gas or butane) at 700-800° C. in a bum box resulting in the formation primarily of hydrogen fluoride (HF), carbon dioxide (CO2) and water. In addition to the high heat requirements and the need for a fuel gas, the conventional treatment method suffers from corrosion problems since the formed HF is highly corrosive at such high temperatures.
An alternative thermal process for destroying F2 involves reacting the F2 gas with steam in the presence of an oxidation source (e.g., air) (see Flippo et al., “Abatement of Fluorine Emissions Utilizing an ATMI CDO™ Model 863 with Steam Injection” (http://www.semiconductorsafety.org/meetings/proc2001/20.pdf)). According to this article, the steam acts as a reducing agent for reducing the F2 gas into HF.
Treatment of various halogen-containing gases other than F2 via plasma reactions have also been disclosed. For example, U.S. Pat. No. 5,187,344 describes decomposing CFCs or trichloroethylene by reacting the CFC or trichloroethylene with water in the presence of a thermal plasma. U.S. Pat. No. 6,187,072 B1 describes oxidizing PFCs under plasma conditions to produce F2. Grothaus et al., “Harmful Compounds Yield to Nonthermal Plasma Reactor”, Technology Today, (pub. Southwest Research Institute Spring 1996) describes treating NF3 by adding H2 gas and passing the resulting mixture through a pulsed corona non-thermal plasma reactor. The products were said to be F2 and HF.
So-called “point-of-use” plasma abatement of PFCs in semiconductor processing effluent streams has also been described (see, e.g., Vartanian et al., “Plasma Abatement Reduces PFC Emission”, Semiconductor International, June 2000, (hereinafter “Vartanian”) and “Evaluation of a Litmas “Blue” Point-of-use (POU) Plasma Abatement Device for Perfluorocompound (PFC) Destruction”, International SEMATECH, Technology Transfer #98123605A-ENG (1998) (hereinafter “SEMATECH disclosure”). Point-of-use abatement involves placing a high-density plasma source (ne>1012/cm3) in the foreline of a process tool between the turbomolecular and dry pumps. Both Vartanian and the SEMATECH disclosure mention that H2 could be an additive gas in the plasma.
Despite these efforts, a need continues to exist for efficient methods and apparatuses for treating halogen-containing effluent gases that operate at low temperature and atmospheric pressure. Such a need particularly exists for halogen-containing gases that are only marginally soluble in water such as F2.